Conventionally, there have been suggested 1) semiconductive BaTiO3 PTC thermistor devices obtained by doping a rare earth element such as La, Gd and the like into BaTiO3 as a ferroelectric substance; and 2) PTC devices obtained by dispersing conductive carbon black particles as a filler in an organic polymer substance as a matrix (see, Patent Document 1) as devices manifesting a so-called “PTC (Positive Temperature Coefficient)” resistance property showing insulating property at high temperatures and conducting property at low temperatures. And, these have been used in various electric and electronic apparatuses.
These PTC devices had problems as described below: In the above item 1), resistance is too high because it is a semiconductor under low resistance condition; and in the above item 2), a principle is used in which with increase in temperature, an organic polymer as a matrix swells, thereby increasing the distance between carbon black particles as a filler, resulting that the resistance raises up at higher temperatures, and since a response to temperature changes depends on swelling of an organic polymer; thus, a high-speed response to temperature change is poor.
On the other hand, among transition metal oxides, sulfides and molecular conductors are a lot of substances which shows conductor (metal)-insulator transition triggered by temperature change. For example, (V, M)2O3 (M=transition metal element such as Cr and the like), NiS2-xSex, bisethylenedithio-tetrathiafluvalene (hereinafter, abbreviated as “BEDT-TTF” in some cases) salts and the like show such a property, namely, a PTC thermistor property. Thermistors employing these substances are expected to have excellent features such as durability, high speed operation as an electronic switch, and tuning of operating temperature from extremely low temperature to high temperature by precise control of their chemical composition. However, during rising temperature condition i) substances showing positive change in resistance are rare, and even in such a case, ii) small ON/OFF ratio at operating temperature, namely, small difference in resistance at or near operating temperature is drawback.
Patent Document 1: Japanese Patent Application Laid-Open No. 8-19174